Desalination and concomitant carbon dioxide capture yielding liquid carbon dioxide

ABSTRACT

Methods and apparatus for desalination of salt water (and purification of polluted water) are disclosed. Saline (or otherwise polluted) water is pumped to a desalination installation and down to the base of a desalination fractionation column, where it is mixed with hydrate-forming gas or liquid to form either positively buoyant (also assisted buoyancy) or negatively buoyant hydrate. The hydrate rises or sinks or is carried into a lower pressure area and dissociates (melts) into the gas and pure water. In preferred embodiments, residual salt water which is heated by heat given off during formation of the hydrate is removed from the system to create a bias towards overall cooling as the hydrate dissociates endothermically at shallower depths, and input water is passed through regions of dissociation in heat-exchanging relationship therewith so as to be cooled sufficiently for hydrate to form at pressure-depth. The fresh water produced by the system is cold enough that it can be used to provide refrigeration, air conditioning, or other cooling; heat removed from the system with the heated residual water can be used for heating or other applications. In other embodiments, desalination or other purification is carried out in “artificially” or mechanically pressurized vessels, which embodiments may be comparatively mobile. Such pressurized systems can be used to capture carbon dioxide from industrial waste gases and to provide liquid carbon dioxide.

This application is a continuation-in-part of U.S. application Ser. No.09/397,500, entitled “DESALINATION USING POSITIVELY BUOYANT ORNEGATIVELY BUOYANT/ASSISTED BUOYANCE HYDRATE” and filed on Sep. 17,1999; which is a continuation-in-part of U.S. application Ser. No.09/375,410, entitled “LAND-BASED DESALINATION USING POSITIVELY BUOYANTOR NEGATIVELY BUOYANT/ASSISTED BUOYANCY HYDRATE” and filed on Aug. 17,1999; which is a continuation-in-part of U.S. application Ser. No.09/350,906, entitled “LAND-BASED DESALINATION USING BUOYANT HYDRATE” andfiled on Jul. 12, 1999.

FIELD OF THE INVENTION

In general, the invention relates to desalination or other purificationof water using hydrates to extract fresh water from saline or pollutedwater. In particular, the invention relates to desalination orpurification of saline or polluted water using, e.g., industrial wastegas as a source of carbon dioxide; capturing the carbon dioxide from thesource gas by forming carbon dioxide hydrate; and producing liquidcarbon dioxide directly as a desalination byproduct.

BACKGROUND OF THE INVENTION

In general, desalination and purification of saline or polluted waterusing buoyant gas hydrates is known in the art. See, for example, U.S.Pat. No. 5,873,262 and accepted South African Patent Application No.98/5681, the disclosures of which are incorporated by reference.According to this approach to water desalination or purification, a gasor mixture of gases which spontaneously forms buoyant gas hydrate whenmixed with water at sufficiently high depth-induced pressures and/orsufficiently low temperatures is mixed with water to be treated at therelatively deep base of a treatment column. According to priortechnology, the treatment column is located at sea. Because the hydrateis positively buoyant, it rises though the column into warmer water andlower pressures. As the hydrate rises, it becomes unstable anddisassociates into pure water and the positively buoyant hydrate-forminggas or gas mixture. The purified water is then extracted and the gas isprocessed and reused for subsequent cycles of hydrate formation. (Wherethe wet gas may be used for some other purpose, such as power generationnearby, it may prove unnecessary to process the gas and instead to usethe gas in a pass-through mode; in this way, only the small amount ofgas not recovered is an operating cost.) Suitable gases include, amongothers, methane, ethane, propane, butane, and mixtures thereof.

The previously known methods of desalination or purification usingbuoyant gas hydrates rely on the naturally high pressures and naturallylow temperatures that are found at open ocean depths below 450 to 500meters when using pure methane, or somewhat shallower when using mixedgases to enlarge the hydrate stability “envelope,” and the desalinationinstallations are essentially immobile once constructed, being fixed topipelines carrying fresh water to land. In certain marine locations suchas the Mediterranean Sea, however, the water is not cold enough for therequisite pressure to be found at a shallow enough depth; this wouldnecessitate using a much longer column, which is impractical. Moreover,many places where fresh water is at a premium are located adjacent towide, shallow water continental shelves where a marine desalinationapparatus would have to be located a great distance offshore.Furthermore, a fixed installation is somewhat less versatile than amobile installation would be. Additionally, the known methodologies haveall required the hydrate, per se, to be buoyant in order to collect thehydrate and the fresh water released therefrom in an efficient manner.

In addition to using hydrates for desalination, it is also known to usehydrates to capture carbon dioxide from gas mixtures such as power plantemissions formed by burning fossil fuels by selectively forming carbondioxide hydrates and then disposing of the carbon dioxide hydrates in anenvironment where the hydrate remains stable, e.g., at the bottom of theocean. See, for example, U.S. Pat. Nos. 5,660,603, 5,562,891, and5,397,553. Although such deep-sea disposal of carbon dioxide in the formof hydrate might be economically feasible where the hydrate is formed ator near the sea (e.g., on an oil rig or aboard a ship or at a seasidefactory), it is far less economically feasible when the hydrate isproduced inland. This is because of the expense of transporting thecarbon dioxide-containing hydrate to the disposal location, thesubstantial majority of the weight and volume of which hydrate consistsof water. (Carbon dioxide hydrate, like methane hydrate and other type Ihydrates contains about 85% water on a molecular basis. In other words,about 85% of the molecules in these hydrates are water and about 15% ofthe molecules are gas molecules. The exact proportions vary slightly andare related to the degree of occupation of the ‘guest’ lattice sites inwhich the gas molecules reside.) Thus, the cost of such deep-seadisposal of carbon dioxide-bearing hydrate is increased substantiallydue to the cost of transporting (unnecessarily, as demonstrated by thepresent invention) the additional weight and volume of the water in thehydrate. Additionally, the previously known teachings of disposing ofcarbon dioxide via carbon dioxide hydrate completely ignores andtherefore fails to take advantage of the tremendous capacity to obtaindesalinated or otherwise purified water by means of forming and thenmelting (i.e., causing to dissociate) the carbon dioxide hydrates.

SUMMARY OF THE INVENTION

The various inventions disclosed herein overcome one or more of thelimitations associated with the prior art and greatly expand the use andbenefits of the hydrate desalination fractionation method by providingfor land-based or mobile installation-based desalination of seawater (orother purification of polluted water) that is supplied to theinstallation and using either positively or negatively buoyant hydrate.The methods of the invention can be employed where input water is toowarm or where suitably deep ocean depths are not available withinreasonable distances for ocean-based desalination to be performed usinggas hydrate, and may be carried out using a gas or gas mixture or even aliquid which produces either positively or negatively buoyant hydrate.Additionally, the invention can be practiced using carbon dioxideobtained from, e.g., industrial exhaust gases, thereby simultaneouslyproviding purified water and capturing the carbon dioxide in the mostefficient form for disposal (or other use, if so desired).

The inventive methods entail cooling the seawater to sufficiently lowtemperatures for hydrate to form at the bottom of a desalinationfractionation column at pressure-depths and temperatures appropriate forthe particular hydrate-forming material being used. A preferredembodiment capitalizes on the property of the hydrate that the amount ofheat given off during formation of the hydrate at depth is essentiallyequal to the amount of heat absorbed by the hydrate as it disassociates(melts) back into pure water and a hydrate-forming material. Inparticular, as liquid or gas forms hydrate, and as the hydrate crystalsrise through the water column (either due to inherent buoyancy of thehydrate or “assisted” by gas trapped within a hydrate mesh shell) andcontinue to grow, heat released during formation of the hydrate willheat the surrounding seawater in the column. As the hydrate rises in thewater column and pressure on it decreases, the hydrate dissociatesendothermically—the hydrate formation is driven primarily by theincreased pressure at depth—and absorbs heat from the surrounding watercolumn. Ordinarily, the heat energy absorbed during dissociation of thehydrate would be essentially the same heat energy released duringexothermic formation of the hydrate such that there would be essentiallyno net change in the amount of heat energy in the system.

According to the invention, however, heat energy that is liberatedduring formation of the hydrate is removed from the system by removingresidual saline water from the water column, which residual saline waterhas been heated by the heat energy released during exothermic formationof the hydrate. Because formation of the hydrate is primarily pressuredriven (as opposed to temperature driven), the hydrate becomes unstableunder reduced pressures as it rises through the water column, and itdissociates endothermically. Because some heat energy released duringexothermic crystallization has been removed from the system, the hydratewill absorb heat from other sources as it melts, thereby creating acooling bias. The preferred embodiment of the invention capitalizes onthis cooling bias by passing the source water through the dissociationregion of the water column, in heat-exchanging relationship therewith,so as to cool the source or supply water to temperatures sufficientlylow for hydrate to form at the base of the installation.

As noted above, the invention may be practiced using liquid, gas, or gasmixtures which produce either positively buoyant hydrate or negativelybuoyant hydrate. In the case of positively buoyant hydrate, the hydratecrystals themselves are positively buoyant and will rise naturally uponformation, upwardly through a desalination fractionation column at thetop of which the hydrate disassociates into fresh water and the gas orgas mixture. In the case of negatively buoyant hydrate, on the otherhand, the hydrate crystals, per se, are denser than the surroundingseawater and therefore ordinarily would tend to sink. By controlling theinjection of the gas (or gas mixture) or liquid which produces thehydrate such that hydrate formation is incomplete, bubbles of the gas orthe less-dense-than-seawater liquid are trapped within a mesh shell ofhydrate, and overall positive buoyancy of the shell will cause thehydrate to rise within the water column.

Preferably, the rising assisted-buoyancy hydrate (negatively buoyanthydrate intimately intermixed with positively buoyant gas or liquidbubbles) is diverted laterally over a “catch basin” so that the hydratedoes not fall back down to the formation portion of the desalinationfractionation column once the mesh shell disintegrates duringdissociation. Solid, negatively buoyant hydrate, which has settled to acatch sump at the base of the apparatus, is pumped to the top of thecatch basin, where it dissociates into gas and fresh water. (If sodesired, forming the negatively buoyant hydrate in a slightly differentmanner will cause all the hydrate to settle in the sump, from which itis pumped to the dissociation/heat exchange catch basin.)

In alternative embodiments of the invention, the input water may or maynot be passed through the dissociating hydrate in heat-exchangingrelationship therewith to be cooled. In either case, the input water is(further) cooled using other, artificial means of refrigeration, thedegree to which such cooling is necessary being in part a function ofthe buoyant or non-buoyant nature of the hydrate. Some heat energy isremoved from the system by removing warmed water which has circulatedaround the desalination fractionation column in a water jacket and whichhas been heated by heat released during hydrate formation.

In the various embodiments of the invention, the purified water will beextremely cool. Advantageously, this cooled water, which preferably willbe used as potable water, can itself be used as a heat sink to providecooling, e.g., refrigeration as a basis for air conditioning in hotclimates.

An additional advantage of land-based desalination or water purificationaccording to the invention is that the installation is not subject todisturbances caused by foul weather and bad sea conditions nearly to thesame extent as a marine site might be. Additionally, access to aninstallation on land is far easier than access to a marine-basedinstallation. Gas handling and storage facilities are more practicableon land, where there is more space and a more secure engineeringenvironment available. Construction is easier on land, and security maybe improved as compared to a marine-based installation.

Moreover, because considerable amounts of residual seawater may beextracted from the system (to remove heat energy from the system), thehydrate slurry will be concentrated. This means that there will be lesssaline water in the upper, dissociation regions of the dissociationfractionation column, and therefore there will be less residual seawaterfor the hydrate to mix with as it dissociates. Thus, less salt will bepresent to contaminate the fresh water produced by dissociation of thehydrate.

Furthermore, because the residual seawater preferably is recirculatedthrough the desalination fractionation column one or more times, othercomponents such as trace elements which are in the seawater (e.g., gold)may be concentrated so that recovery from the seawater becomespractical. Additionally, the concentrated seawater may itself be usefulor desirable. For example, marine aquarists might purchase suchconcentrated seawater to use for mixing replacement water for theiraquaria, and such concentrated seawater would facilitate recreating thespecific microcosm from which it was extracted.

In further embodiments, the desalination installation is constructed sothat the hydrate dissociation occurs while the hydrate is still at somedepth, such that it is still under considerable pressure, and thehydrate-forming gas is captured at this depth and processed for re-usewhile still at such pressures. Considerable efficiencies of operationare obtained with such arrangements.

In still further embodiments of the invention, rather than relying onthe weight of a long column of water to create pressures appropriate forhydrate formation, the inventive methods may be practiced usingself-contained, sealed hydrate formation/separation and hydratedissociation/heat exchange vessels, which vessels are mechanicallypressurized using appropriate hydraulic pumping systems. Suchmechanically pressurized installations can be made comparatively mobileand may be used to provide the benefits of the invention in highlydiverse settings. Moreover, such mechanically pressurized systems can beused to control the dissociation of the hydrate so that, when usingcarbon dioxide as the hydrate-forming medium, for example—particularlywhen the carbon dioxide is provided by means of and is to be capturedand sequestered from exhaust gases (e.g., industrial source exhaustgases)—the carbon dioxide released upon hydrate dissociation can be ineither the liquid state or the gaseous state.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail in connection withthe drawings, in which:

FIG. 1 is a generalized, diagrammatic depiction of a land-baseddesalination installation;

FIG. 2 is a diagrammatic, side elevation view of an embodiment of adesalination fractionation column which utilizes positively buoyanthydrate and which may be employed in the installation shown in FIG. 1;

FIGS. 3 and 4 are diagrammatic, side elevation views showing twoalternative heat extraction portions of a desalination fractionationcolumn employed in the installation shown in FIG. 1;

FIG. 5 is a diagrammatic, side elevation view of another embodiment of adesalination fractionation column which utilizes positively buoyanthydrate and which may be employed in the installation shown in FIG. 1;

FIG. 6 is a diagrammatic, side elevation view showing overlapping watervents used in the desalination fractionation column shown in FIG. 5;

FIG. 7 is a diagrammatic, side elevation view of yet another embodimentof a buoyant hydrate-based desalination fractionation column employed inthe installation shown in FIG. 1, which embodiment is similar to thatshown in FIG. 5.

FIG. 8 is a diagrammatic, side elevation view of an embodiment of adesalination fractionation column which permits the utilization ofnegatively buoyant hydrate and which may be employed in the installationshown in FIG. 1;

FIGS. 9 and 10 are schematic, isometric and end views, respectively, ofthe dissociation and heat exchange portion of the desalinationfractionation column shown in FIG. 8;

FIG. 11 is a Pressure/Temperature diagram depicting regions of CO₂hydrate stability, the CO₂ liquidus, and the operating envelope for anegatively buoyant, CO₂ hydrate-based desalination system;

FIG. 12. is a diagrammatic, side elevation view of a residual fluidreplacement section designed to facilitate washing of the hydrateslurry;

FIG. 13 is a diagrammatic, side elevation view of another embodiment ofa desalination fractionation column which permits the utilization of anegatively buoyant hydrate, which embodiment facilitates separation ofresidual seawater from the negatively buoyant hydrate;

FIG. 14 is a diagrammatic, side elevation view of a slurry holding,fluid separation apparatus used in the installation of FIG. 13;

FIG. 15 is a diagrammatic, side elevation view of an embodiment of adesalination fractionation column configured to maintain thehydrate-forming gas at elevated pressure;

FIG. 16 is a diagrammatic, side elevation view of an embodiment of amechanically pressurized desalination system configured to usepositively buoyant hydrate;

FIG. 17 is a diagrammatic, side elevation view of an embodiment of amechanically pressurized desalination system which is similar to thatshown in FIG. 16 but which is configured to use negatively buoyanthydrate;

FIG. 18. is a diagrammatic, side elevation view of an embodiment of amechanically pressurized desalination system configured to use eitherpositively or negatively buoyant hydrate;

FIG. 19 is a generalized, diagrammatic depiction of a mechanicallypressurized desalination system located on a ship;

FIG. 20 is schematic diagram illustrating the components of a systemconfigured to produce liquid carbon dioxide and fresh water usingindustrial exhaust gas as the supply of carbon dioxide to form hydrate;

FIG. 21 is a phase diagram illustrating the relativepressure/temperature conditions at which hydrogen sulfide and carbondioxide hydrate form;

FIG. 22 is a schematic diagram illustrating a dissociation vessel inwhich carbon dioxide hydrate is allowed to dissociate so as to produceliquid carbon dioxide and fresh water;

FIG. 23 is a phase diagram illustrating P-T pathways along which carbondioxide hydrate is allowed to dissociate in the dissociation vesselshown in FIG. 22; and

FIG. 24 is a graph illustrating the variation in density of liquidcarbon dioxide with varying pressure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A land-based desalination installation is shown schematically in FIG. 1in generalized fashion. The installation may be divided roughly intothree sections or regions: an intake portion 10; a water purificationportion 12; and post-processing and downstream usage section 14.

The intake portion 10 consists essentially of the apparatus and varioussubinstallations necessary to extract seawater from the ocean 16 andtransport it to the desalination/purification installation at region 12,including subaquatic water intake piping 18 and pumping means (notshown) to draw the water from the ocean and pump it to shore forsubsequent processing. Large volume installations can be locatedrelatively close to the sea to reduce the piping distance of the inputwater to a minimum, and establishing the installation as close to sealevel as possible will reduce the cost of pumping against pressure head.

The intake pipeline 18 preferably extends sufficiently out to sea thatit draws deep water, e.g., from the slope 20 of the continental shelfbecause deep water is more pure and colder than shallow water.Alternatively, water may be drawn from locations closer to land, e.g.,from areas on the continental shelf 22 where the distance across theshallow water is too great for practice. The precise depth from whichwater is drawn will ultimately be determined by a number of factors,including primarily the specific embodiment of the desalinationfractionation column which is employed, as described below. Ideally, thedesalination installation, per se, is located so that the highest partof the fluid-handling system is at or below sea-level to reduce thecosts of intake pumping.

Additionally, the water may be pretreated at a pretreatment station 24.Pretreatment consists mainly of deaeration, filtering to removeparticulate matter and degassing, consistent with the requirement thatmaterial necessary for hydrate nucleation and growth not be removed fromthe water.

A preferred embodiment of the purification installation 30, per se, isillustrated in FIGS. 2, 3, and 4, which embodiment utilizes positivelybuoyant hydrate to extract fresh water from seawater. Seawater is pumpedinto the installation 130 at water input 32 and is pumped down to thelower, hydrate formation section 34 of the installation. The bottom ofthe hydrate formation section is no more than about 800 meters deep, andperhaps even shallower (again depending on the particular gas or gasmixture being used). A suitable, positively buoyant hydrate-forming gas(or liquid) is injected into the hydrate formation section at 36, andpositively buoyant hydrate 38 spontaneously forms and begins to risethrough the water column, as is known in the art.

The hydrate-forming gas can be pumped using sequential, in-line,intermediate pressure pumps, with the gas conduit extending either downthrough the fractionation column, per se, or down through the inputwater line so that gas line pressure is counteracted by ambient waterpressure. As a result, it is not necessary to use expensive, highpressure gas pumps located on the surface. Alternatively, once a gas hasbeen liquefied, it can be pumped to greater depths without furthersignificant compression.

Hydrate formation (crystallization) is an exothermic process.Accordingly, as the positively buoyant hydrate forms and risesautomatically through the water column—forming a hydrate “slurry” ashydrate crystals continue to nucleate and grow as they rise, until thehydrate-forming gas is used up—the surrounding water, which willincreasingly become a concentrated saline “residue,” will be heated bythe heat energy released during crystallization of the hydrate.

Below a certain salinity, the heated residual seawater will have arelatively decreased density and will rise in the column along with thehydrate 38. When the salinity of the residual seawater rises high enoughdue to the extraction of fresh water from it, however, the highly salineresidual seawater will sink to the bottom of the water column. Thishighly saline residual seawater is collected in sump 40 at the bottom ofthe fractionation column and is removed.

As the slurry of hydrate and heated residual seawater rises in thefractionation column, heated residual seawater is removed from thesystem in heat extraction portion 44 of the fractionation column at oneor more points 46. The heat extraction section 44 is shown in greaterdetail in FIG. 3. As illustrated in FIG. 3, for one mode of separationof hydrate and slurry, water is pumped from the system as part of thevertical fractionation process. This is accomplished through a two-stageprocess. An internal sleeve 45 allows a primary separation to takeplace, as a water trap 49 is formed below the top of the sleeve. Hydratecontinues to rise, while water floods the entire section 44. Water ispumped from below the level at which hydrate exits from the top of thesleeve through fine conical screens 47. These are designed to obstructthe passage of particulate hydrate. (The screens can be heatedperiodically to clear them of hydrate when flow restriction exceedsdesign limits.) Residual water is drawn off at a slow enough rate thatany hydrate that may reside within water drawn toward the screen has agreater tendency to rise buoyantly than the tendency toward downwards orsideways movement associated with the force of suction of the drawn-offwater. Very buoyant gas rises and stays within the column.

An alternative configuration 44′ of the heat extraction zone is shown inFIG. 4. In this configuration, a centrifuge is used to allow a separate,mechanically-driven density fractionation system to operate. In thisconfiguration, a segment 51 of the column is made mobile and capable ofrotary movement. The mobile, rotary centrifuge column segment is carriedby bearings 53 at the base 55 and at intervals along its height to keepit in vertical alignment with the entirety of the column, and to allowit to rotate with respect to the portions 57, 59 of the column above andbelow it. This section is motor-driven, using a hydraulic system 61driven from the surface. Vanes 63 within the centrifuge section willcause the water column to rotate, which vanes are designed based onturbine vane design to cause the hydrate-residual water in the sectionto rotate without turbulence and with increasing velocity toward the topof the section where residual water is extracted. Gravity “settling” orfractionation works here in a horizontal plane, where the heavierresidual water “settles” toward the sides of the column while thelighter, more buoyant hydrate “settles” toward the center of the column.The hydrate continues to rise buoyantly and concentrates in the centerof the centrifuge section. It will be appreciated that more than onesuch centrifuge section may be employed.

As the hydrate rises into the upper, dissociation and heat exchangeregion 50 of the desalination fractionation column, the depth-relatedpressures which forced or drove formation of the hydrate dissipate;accordingly, the hydrate, which is substantially in the form of aslurry, will be driven to dissociate back into the hydrate-forming gas(or mixture of gases) and fresh water. However, regardless of theparticular method used to extract the warmed residual seawater, heatenergy in the surrounding seawater which ordinarily (i.e., in the priorart) would be absorbed by the hydrate as it dissociates is no longeravailable to the hydrate. Therefore, because heat has been removed fromthe system by extracting warmed residual seawater in the heat extractionportion 44 of the apparatus, a net or overall cooling bias is created inthe upper, dissociation and heat exchange portion 50 of theinstallation.

This cooling bias is capitalized upon to significant advantage. Inparticular, as indicated schematically in FIG. 2, water being pumpedinto the system (at 32) is passed in heat exchanging relationshipthrough the regions of dissociating hydrate. For example, it iscontemplated that the dissociation and heat exchange portion 50 may beconstructed as one or more large, individual enclosures on the order ofone hundred meters across. The input water will pass via a series ofconduits through the regions of dissociating hydrate and will be cooledsignificantly as it does so. In fact, although some initialrefrigeration will be required at start-up of the process, which initialrefrigeration may be provided by heat exchange means 52, theinstallation eventually will attain a steady-state condition in whichthe amount of heat energy transferred from the input water to thedissociating hydrate is sufficient to cool the input water totemperatures appropriate for spontaneous formation of hydrate at theparticular depth of the dissociation column.

Ideally, the input water is stabilized at 4° C. or below. This isbecause below that temperature, the density of the water increases,which enhances separation of the hydrate-water slurry formed byinjections of the gas. Additionally, at a given pressure, hydratenucleation proceeds faster at colder water temperatures. During thestart-up period, the system is run in a mode of maximum warm fluidextraction (to create a state of induced thermal bias) beforeequilibrium or steady-state is reached; although the duration of thisstart-up period will vary depending on the particular installationparameters, the design goal is that once steady-state is reached, thesystem can be run for extremely long operating periods without beingshut down, i.e., periods on the order of years. Controlling residuewater extraction, and thus heat removal, maintains a steady-statecondition so that the apparatus does not keep cooling to belowsteady-state operating conditions.

Once the hydrate has dissociated into its constituent fresh water andgas or gases, the fresh water is pumped off, e.g. as at 54, and the gasis captured and recycled. (Provisions may be made for liquefying certaingases where this is desired.) Additionally, a portion of the water inthe dissociation and heat exchange region 50 will be “gray water,” whichis fresh water containing some small portion of salts that have beenremoved from the hydrate by washing of the hydrate with water. Thedistinction between the “gray” or mixed water and pure fresh water isindicated schematically by dashed line 56. The gray water may besuitable for drinking, depending on the salt concentration, or foragricultural or industrial use without further processing. The cold,gray water may be recycled back into the fractionation column, either bypumping it back down to the hydrate formation section 34, as indicatedat 58; or it may be injected back into the concentrated hydrate slurryat a region of the fractionation column located above the heatextraction portion 44, as indicated at 60, to increase the fluid natureof the hydrate slurry and to aid in controlling overall thermal balanceof the system. Furthermore, providing gray water at 62 to diluteresidual interstitial fluid allows for pre-dissociation washing.

As further shown in FIG. 1, in the post-processing and downstream usagesection 14, the fresh water preferably is treated by secondary treatmentmeans 64. The secondary treatment means may include, for example, finefiltering, gas extraction, aeration, and other processing required tobring the water to drinking water standard.

Moreover, it is extremely significant that depending on operatingparameters such as temperature of the source water, the amount ofresidual seawater extracted in the heat extraction section 44,dimensions of the installation, and other parameters such as viscositiesof fluids within the system; buoyancy of the hydrate relative to allfluids within the system; salinity and temperature of residual water;the design output requirements of fresh water; salinity and temperatureof input water; design cooling requirements; system inefficienciesaffecting thermal balance; etc., the fresh water produced will besignificantly cooled. This cooled water can be used to absorb heat fromother applications or locations such as the insides of buildings, andhence can be used to provide refrigeration or provide forair-conditioning.

Finally, once the seawater has been cycled through the desalinationfractionation column and downstream processing applications a desirednumber of times, the residual, concentrated seawater (which may behighly saline in nature) is simply pumped back to sea. Alternatively, itmay be retained for those who desire it.

With respect to overall design, engineering, and constructionconsiderations for the system, it is contemplated that the desalinationfractionation column 130 will be on the order of 15 to 20 meters indiameter, or even larger. Conventional excavation and shaft-liningmethodologies common to the mining and tunneling industry can be used inthe construction of the column 130. Overall dimensions will bedetermined based on the total desired fresh water production desired andrelevant thermodynamic considerations. For example, one cubic meter ofmethane hydrate has the capacity to warm about 90 to 100 cubic meters ofwater by about 1° C. as it forms, and that same cubic meter of hydratehas the capacity to cool about 90 to 100 cubic meters of water by about1° C. as it dissociates. (Mixes of suitable gases have higher heats offusion, which makes the process more efficient.) Required coolingtherefore will, in part, determine hydrate production rates, and hencedimensions of the system and the choice of gas or gases to meet thoseproduction rates.

Preferably, the diameter of the residual fluid removal column segment islarger. This facilitates buoyant, upward movement of the hydrate throughthe water column while first allowing separation of residue water fromthe hydrate in the heat extraction region 44, and then dissociation andheat exchange in the dissociation and heat exchange region 50.

The dissociation and heat exchange region 50 may be constituted not justby a single dissociation “pool,” as shown schematically in FIG. 2, butrather may consist of a number of linked, heat-exchanging devices in anumber of different water treatment ponds or pools. The actual depth,size, throughput, etc. will depend on the production rate, which willdepend, in turn, on the temperature of the input water, the particulargas or gas mixture used to form the hydrate, the rate at which heat canbe removed from the system, etc.

The input of water into the base of the fractionation column can becontrolled by a device (not shown) that alters the input throat diameterso as to facilitate mixing of the gas and water, thereby promoting morerapid and complete hydrate formation. Alternatively or additionally,hydrate formation can be enhanced by creating flow turbulence in theinput water, just below or within the base of the hydrate forming gasinjection port 36. It may further be desirable to vary the diameter ofthe desalination fraction column in a manner to slow the buoyant descentof the hydrate slurry, thereby enhancing hydrate formation.

The dissociation and heat exchange region 50 will be significantly widerand larger than the lower portions of the desalination column. This isbecause hydrate will be floating up into it and dissociating into gasand fresh water at a rate that is faster than that which could beaccommodated in a pool that is the diameter of the column itself.Moreover, the requirement for heat will be great; if sufficient heatcannot be provided, water ice will form and disrupt the desalinationprocess. Provision for physical constriction within a column will holdhydrate below the level where it dissociates freely, thus providing fora control on the amount of gas arriving at the surface. This is done forboth normal operational and safety reasons.

Because the positively buoyant hydrate used in this embodiment of theinvention floats, fresh water tends to be produced at the top of thesection, thereby minimizing mixing of fresh and saline water. To inhibitunwanted dissociation, the heat exchanger apparatus may extend downwardto the top of the residual water removal section. The dissociation andheat exchange pools do not need to be centered over the water column;moreover, more than one desalination fractionation column may feedupward into a given dissociation and heat exchange pool. Similarly,groups of desalination fraction columns can be located close together soas to be supported by common primary and secondary water treatmentfacilities, thereby decreasing installation costs and increasingeconomy.

In addition to large-scale installations, relatively small-scaleinstallations are also possible. For these installations, smallerdiameter desalination columns can be constructed in locations wherelower volumes of fresh water are required. Although overall efficiencyof such systems will be lower than larger scale systems, the primaryadvantage of such small-scale installations is that they can beimplemented using standard drilling methods. Furthermore, mass-produced,prefabricated desalination apparatus sections can be installed in thecasings of drilled holes; this allows the installation to be completedin a relatively short period of time. Capital cost of such aninstallation also is reduced, as fabrication of the components can becarried out on an industrialized basis using mass production techniques.The various operating sections of a smaller-scale installation might bereplaced by extracting them from their casing using conventionaldrilling and pipeline maintenance techniques.

An alternate, slightly simplified embodiment 230 of a desalinationfractionation column according to the invention is shown in FIG. 5. Inthis embodiment, hydrate formation occurs essentially within a thermalequilibration column 132. The thermal equilibration column 132 has anopen lower end 134 and is suspended in shaft 136. In this embodiment,input water is injected near the base of the desalination column 132,e.g. as at 138, preferably after passing through heat exchange anddissociation region 150 of the column 230 in similar fashion to theembodiment shown in FIG. 2. Positively buoyant hydrate-forming gas isinjected into the lower portions of the thermal equilibration column132, as at 140, and hydrate will form and rise within the column 132much as in the previous embodiment. The embodiment 230 is simplified inthat heat of formation of the hydrate is transferred to watersurrounding the thermal equilibration column 132 within a “water jacket”defined between the walls of the column 132 and the shaft 136 in whichthe desalination fractionation column is constructed. To this end, thehydrate formation conduit preferably is made from fabricated (i.e.,“sewn”) artificial fiber material, which is ideal because of its lightweight and its potential for being used in an open weave that greatlyfacilitates thermal equilibration between residual saline water withinthe thermal equilibration column 132 and seawater circulating within thewater jacket.

As is the case with the embodiment shown in FIG. 2, warmed water ispumped out of the system, this warmed water being water which hascirculated within the water jacket. In contrast to the embodiment shownin FIG. 2, however, the intent of removing warmed water from the waterjacket is not to remove so much heat energy that the input water isautomatically cooled to temperatures suitable for formation of thehydrate at the base of the column, but rather it is simply to removeenough heat energy to prevent water within the interior of the hydrateformation conduit from becoming so warm that hydrate cannot form at all.Accordingly, the rate at which warm water is removed from the waterjacket may be relatively small compared to the rate at which warm wateris removed from the heat extraction portion 44 of the embodiment shownin FIG. 2. As a result, it is necessary to supplement the cooling whichtakes place in the heat exchange and dissociation region 150 usingsupplemental “artificial” refrigeration means 152. Operation isotherwise similar, to that of the embodiment shown in FIG. 2: freshwater is extracted from the upper portions of the heat exchange anddissociation portion 150; “gray water” is extracted from lower portionsof the heat exchange and dissociation region 150, i.e., from below theline of separation 156; and concentrated brine is removed from brinesump 141.

To facilitate “settling out” of brine which is sufficiently dense to benegatively buoyant due to concentration and/or cooling, and tofacilitate heat transfer and thermal equilibration, the equilibrationcolumn 132 preferably is constructed with overlapping joints, as shownin FIG. 6. This configuration permits the buoyant hydrate to risethroughout the column, while cooled, more saline water can flow outthrough the vents 142, as indicated schematically.

The desalination fractionation column installation may be furthersimplified by feeding the input water into the system without passing itthrough the dissociation section 250 of the embodiment 330 shown in FIG.7. If the input water is not sufficiently cold, more artificialrefrigeration will need to be provided by refrigeration means 252, butoperation is otherwise the same as embodiment 230 shown in FIG. 5.

Whereas the embodiments described so far utilize gas or mixtures of gaswhich form positively buoyant hydrates under appropriate temperature andpressure conditions, the versatility of hydrate-based desalination orpurification can be expanded greatly by adapting the methods andapparatus described above to accommodate negatively buoyant hydrates. Anembodiment 430 of a desalination fractionation “column” configured topermit the use of negatively buoyant hydrate for water purification isshown in FIGS. 8-10. The major difference between this embodiment 430and the preceding embodiments of desalination fractionation columns isthat the heat exchange and dissociation portion 350 of the installationis laterally or horizontally displaced or offset relative to the hydrateformation and heat removal sections 336 and 346, respectively. Thehydrate formation and heat removal sections are similar to those in theembodiments described above.

A number of different operating gases can be employed with thisconfiguration. Low molecular weight gases such as O₂, N₂, H₂S, Ar, Kr,Xe, CH₄, and CO₂ can all form hydrates under differentpressure-temperature conditions. Each of the different hydrate-forminggas systems will require special design of the hydrate column which istailored to the particular gas used in the installation, but theprinciples of hydrate formation to extract fresh water will remain thesame. Additionally, adding small amounts of additive gas(es) to theprimary hydrate-forming gas may broaden the hydrate stability field inthe same way the methane hydrate stability field is expanded by mixinghigher density hydrocarbon gases with methane.

Although a number of different gases that form negatively buoyanthydrate may be used for hydrate desalination, carbon dioxide and thedesalination column in which it is used are described herein toillustrate the design requirements and considerations for a desalinationsystem employing hydrate that is naturally less buoyant than seawater.Carbon dioxide (or gas mixtures containing predominantly carbon dioxide,referred to herein simply as “carbon dioxide” for simplicity) is anideal gas to use for a number of reasons. In particular, carbon dioxidedoes not combust under the physical and thermal conditions encounteredin the hydrate desalination apparatus, and is thus virtuallyhazard-free. Carbon dioxide hydrate is stable at shallower depths thanmethane hydrate (and about the same as mixed gas methane hydrate). Evenif present dissolved in relatively high concentrations, carbon dioxideis safe for human consumption and is not offensive to either taste orsmell (as would be the case of H₂S hydrate) (In fact, fresh waterproduced using carbon dioxide can be made so as to retain some quantityof the carbon dioxide, thereby providing soda water that is similar tomany popular brands but that is different in at least one significantway in that it will contain all the naturally occurring minerals foundin seawater in proportion to the remaining salts not removed during thedesalination process.) Carbon dioxide hydrate is, like methane,tasteless and odorless. There is considerable recent experimentalinformation which demonstrate clearly the actual marine behavior of theformation and behavior of carbon dioxide hydrate. Carbon dioxide is verycommon and can be produced locally almost anywhere and is also commonlyavailable as an industrial waste product—particularly in the exhaustgases produced when burning fossil fuel. (A further advantage of usingcarbon dioxide as compared to methane or methane mixes is that thehigher heat of fusion of carbon dioxide hydrate will heat the residualwater more quickly than methane or methane-mixed gases; thus, theinduced thermal bias will be higher and the system will operate moreefficiently.)

Design and engineering of the desalination fractionation column will bedetermined in large measure based on the phase properties of theparticular gas being used. FIG. 11 shows, for example, the carbondioxide hydrate stability regions superimposed over the carbon dioxidephase diagram. The shaded portion of the diagram indicates that carbondioxide hydrate (formed from carbon dioxide gas) is stable at from anupper pressure limit of about 18 atmospheres, just above 0° C., to about40 atmospheres pressure at just above about 8° C. With respect to carbondioxide, per se, the liquidus extends from about 37 atmospheres pressureat just above 0° C., to about 40 atmospheres pressure at just above 8°C. Above the liquidus, carbon dioxide exists as a gas; below theliquidus, carbon dioxide spontaneously compresses to a liquid.

Accordingly, the system is constructed so that, assuming carbon dioxideis used as the operating gas, the carbon dioxide is injected into thehydrate formation portion of the column at ambient temperature andpressure that is within the operating region 450 that consists of theportion of the carbon dioxide hydrate stability zone that lies above thecarbon dioxide liquidus and above the freezing point of water. Thepractical result of this arrangement is that the range of water depthsat which carbon dioxide may be used as the operating gas is relativelysmall and is comparatively shallow. Accordingly, a relatively shallowland apparatus can be constructed, which will reduce constructioncomplexity and cost.

Similar to the embodiments described above, carbon dioxide (or othernegatively buoyant hydrate-forming gas, as desired) is injected near thebase of the hydrate formation section 336 (e.g., at 352) and mixed withsupply or input seawater that has been chilled by being passed throughthe heat exchange and dissociation portion 350 and/or by “artificial”refrigeration, as at 354. The carbon dioxide hydrate will float only ifthe formation of the hydrate is incomplete such that a complex,hydrate-gas meshwork is formed. This condition is met when the gas isinjected rapidly and in relatively large bubbles. The carbon dioxidehydrate isolates carbon dioxide gas bubbles from the surroundingseawater, thereby preventing further formation of hydrate. The combinedgas/liquid carbon dioxide and hydrate is positively buoyant, even thoughthe hydrate per se is negatively buoyant (i.e., has a greater specificgravity than the seawater), and floats upward, as at 356. Additionally,some of the bubbles will burst and new hydrate shells will be formed;hydrate shells with gas bubbles predominantly form new carbon dioxidehydrate rims, which are assisted upward by carbon dioxide gas whichtends to adhere to solid hydrate particles.

The system is designed to produce as much hydrate as possible,consistent with leaving enough warm, lower-density, residual fluid toform a “flux” and to allow extraction of heat by removing the residualseawater in the heat extraction section 346. The system furthermore hasthe capacity for very rapid liquid or gas injection, which may be intime-sequence bursts rather than being continuous. It is intended thatnot all gas form hydrate, as noted above, to ensure incomplete formationof hydrate. Thus, larger quantities of gas are required for a negativelybuoyant hydrate-based system than for a complete hydrate-forming gassystem such as the positively buoyant hydrate-based systems describedabove.

As in the case of positively buoyant hydrate-based embodiments,formation of the negatively buoyant (assisted buoyancy) hydrate isexothermic. Accordingly, heat which is given off during hydrateformation warms the surrounding, residual seawater, which makes theresidual seawater more buoyant than the chilled seawater which is beinginput into the lower part of the column. The residual seawater thereforemoves buoyantly upward along with the hydrate as new, denser input wateris supplied to the base of the fractionation column, as at 360.

The upward movement of the surrounding residual seawater, along with theoriginal upward movement of the assisted buoyancy hydrate, has a certainmomentum associated with it. This carries the hydrate upward through thecolumn until it reaches a lateral deflection zone 362, where thehydrate/residual seawater slurry is diverted horizontally or laterallyrelative to the hydrate formation and heat removal sections 336 and 346and into the dissociation and heat removal section 350. Thus, eventhough some of the hydrate “bubbles” will burst or crack, therebyreleasing the carbon dioxide gas contained therein and losing buoyancy,the hydrate in large measure continue to move upward and over into theheat exchange and dissociation region of the column 350 due to thismomentum. As the hydrate loses momentum within the heat exchange anddissociation portion 350, it will settle and dissociate into the gas andfresh water, which will separate from residual seawater as described ingreater detail below.

Some of the hydrate, however, will form solid masses without entrappedgas and will sink to the lowermost, sump portion 364 of the column.Concentrated brine will also sink to and settle in the sump portion 364.The sunken hydrate and concentrated residual brine are pumped out of thesump at 365 and separated by appropriately configured separation means366. The waste saline water 368 is disposed of as appropriate, and aslurry consisting of the sunken hydrate is pumped upwardly as indicatedat 370 and is discharged into the heat exchange and dissociation chamber350, e.g. at 372, where the hydrate dissociates into gas and freshwater.

Within the dissociation and heat exchange chamber 350, the hydrate,whether delivered or transported to the chamber via the lateraldeflection portion 362 of the column or pumped from the sump of thedesalination fractionation column 364, will dissociate into fresh waterand the hydrate-forming gas.

To facilitate separation of fresh water from saline water, it isnecessary to promote transfer of as much hydrate to the upper part ofthe dissociation and heat exchange chamber 350 as possible; to holdhydrate as high in the dissociation and heat exchange chamber 350 aspossible until dissociation of that volume of hydrate is complete; andto keep mixing of the fresh water produced by dissociation and the moresaline residual water to a minimum. The configuration of thedissociation and heat exchange chamber shown in FIGS. 9 and 10facilitates these objectives.

In particular, the assisted buoyancy hydrate slurry rising through thedesalination fractionation column enters the chamber as at 360 afterbeing diverted laterally at deflection portion 362, as indicatedschematically in FIG. 9. Additionally, hydrate slurry being pumped fromthe sump is injected into the dissociation chamber at 372, where it maybe placed within special fluid separation devices. The dissociation andheat exchange chamber is constructed with a number of canted separatorshelves 380 which extend from one end of the chamber to the other, aswell as from one side of the chamber to the other. The canted nature ofthe shelves allows the denser saline water to sink and the lighter freshwater to rise within and between the shelves, thereby minimizingturbidity and mixing of saline and fresh water. The separator shelves380 are canted in that they slope downward, both from one end of thechamber to the other as well as from one side of the chamber to theother. The separator shelves have pass-through apertures 382 which allowthe denser, saline water to sink within the system and the less dense,fresh water to rise within the system to the top of the chamber as thehydrate dissociates into the fresh water and gas.

Fresh water, which is cooled due to the cooling bias created by theremoval of warm residual water as described above in connection with thepositively buoyant hydrate embodiments, is removed as at 384 and may beused for cooling as well as for potable water. “Gray” water and salineresidue are removed from lower portions of the heat exchange anddissociation chamber 350, as at 386 and 388, and are handled asdescribed above in the context of the positively buoyant hydrateembodiments, e.g., gray water may be used for drinking or industrialapplications and the saline residue may be recycled back as input intothe base of the desalination fractionation column.

As an alternative to gaseous carbon dioxide, liquid carbon dioxide canbe used to form assisted buoyancy hydrate. At the relatively shallowdepths appropriate to the formation of hydrate for separation of freshwater, liquid carbon dioxide is more buoyant than seawater (although notas buoyant as gaseous carbon dioxide.) By injecting liquid carbondioxide energetically into seawater, a resultant meshwork of hydrate andliquid carbon dioxide is formed which is positively buoyant. Themeshwork mass will rise spontaneously as a whole immediately uponforming and will behave essentially the same as a hydrate meshworkformed from gaseous carbon dioxide and carbon dioxide hydrate.

(Advantages of liquid carbon dioxide over gaseous carbon dioxide stemfrom the fact that once the carbon dioxide is compressed, it can betransported to deeper depths without further compression. Thus,injecting liquid carbon dioxide at depths of five hundred meters ormore—well below the liquidus—is possible without the need for deep,in-line pumps. Moreover, deeper (i.e., higher pressure) injection ofliquid carbon dioxide will promote very rapid crystallization and growthof the hydrate crystals.)

When liquid carbon dioxide is used to form assisted buoyancy hydrate,dissociation is comparatively violent because the unhydrated liquidcarbon dioxide trapped within the meshwork produces large volumes ofcarbon dioxide gas when the mixture rises above the liquidus. Thus, inaddition to the carbon dioxide gas released by dissociation of thehydrate (which occurs above the carbon dioxide liquidus), the extra gasproduced by conversion of the liquid carbon dioxide to gaseous carbondioxide has the potential to cause significant turbulence and mixing.Therefore, flow of the hydrate should be controlled such that it entersthe dissociation section while still within the hydrate stability fieldin order to preclude significant dissociation while residualinterstitial saline water remains in the slurry.

Additionally, where carbon dioxide liquid is used to form assistedbuoyancy hydrate, care should be taken to allow residual fluid to alterits state to gas once the hydrate has risen above the liquidus pressuredepth, but while the hydrate remains stable. This will reduce turbulenceand mixing when the hydrate finally dissociates.

Ideally, residual saline water should be replaced by fresh water beforethe hydrate rises into the gas-stable zone and then the dissociationarea of the carbon dioxide hydrate phase diagram (FIG. 11). This can beaccomplished using multiple water injection points alternatinglyarranged between multiple residual or interstitial water removalsections, as illustrated in FIG. 12. In other words, the fluid removalsection 44 (FIG. 2) is constructed as an alternating sequence of freshwater injection subsections 412 and fluid removal subsections 414constructed as shown in either FIG. 3 or FIG. 4. The benefits ofremoving the interstitial saline fluid include additional heat removal;washing of the slurry (i.e., removal of pollutants or adhering ions orparticulate material from the surface of the hydrate crystals) by fluidreplacement; and direct removal of saline interstitial water from thehydrate slurry and dilution or replacement of the original salineinterstitial fluid produced by the process of hydrate formation.

Although washing of interstitial water is strongly recommended for theslurry mixture of liquid carbon dioxide and carbon dioxide hydrate—aswell as for any hydrate being used for water purification, wherepossible—so as to minimize turbulence and mixing attributable to theliquid carbon dioxide converting to gaseous carbon dioxide, washing theslurry and flushing saline interstitial fluid therefrom would alsoprovide benefits for any positively buoyant hydrate-based or assistedbuoyancy hydrate-based system. In particular, injecting cold water(either fresh or gray) from the dissociation section into the hydrateslurry will remove additional heat from the hydrate at the same timethat saline interstitial water is flushed from the hydrate slurry.Moreover, multiple residual water flushings will ensure greater freshwater production.

Another embodiment 530 of a desalination fractionation “column” which isconfigured to utilize negatively buoyant hydrate and which facilitatesseparation of the hydrate and residual seawater is illustrated in FIGS.13 and 14. The “column” is configured as an asymmetric, U-shapedinstallation, which consists primarily of a seawater input conduit 432,a hydrate formation and catch sump region 434, and a residue fluid riserconduit 436. As in previous embodiments, the seawater input conduitpasses through a dissociation and heat exchange region 438 which, inthis embodiment, is configured especially as a hydrate “catch basin.” Asin the previous embodiments, the input water is passed through thedissociation/heat exchange catch basin 438 in heat exchangingrelationship with dissociating hydrate in order to chill the inputseawater.

The input seawater is pumped to the base 440 of the column, where itturns and flows upward and laterally through elbow portion 442 beforeentering the hydrate formation and catch sump 434. Negatively buoyanthydrate-forming liquid or gas is injected into the input seawater in thehydrate formation and catch sump at 444. (Means 945 for liquefyingcertain gases are provided; residual fluid can be used in a heatexchanger 457 to provide cooling for the liquefication process.)Injection of the gas or liquid is controlled such that hydrate formationis complete (in contrast to incomplete, as in the case of the previouslydescribed, assisted buoyancy embodiment), i.e., such that all gas isutilized to form hydrate. The negatively buoyant hydrate settles to thebottom of the catch sump 434. As the hydrate settles, it displaces theresidual seawater, which is warmed by the heat liberated during hydrateformation. The residual seawater therefore rises buoyantly throughresidue fluid riser conduit 436, and it is pumped out of the system toremove heat and create a cooling bias in the system as in the previouslydescribed embodiments.

The rate of formation and settling of the hydrate is controlled suchthat it “packs” down to the point of being grain supported. Mechanicalapparatus such as a vibration tray is located on the sloping floor 439of the settling portion of the hydrate-residual fluid chamber 434. Thisconcentrates the hydrate and minimizes residual fluid remaining so thatthe hydrate can be pumped rapidly, as a slurry, from the base of thesump up into the dissociation/heat exchange catch basin 438 via slurrypumping conduit 446. The hydrate slurry is pumped to thedissociation/heat exchange catch basin 438 at a rate that is generallyfaster than the rate at which positively buoyant hydrates rises in thepreviously described embodiments. Decreasing the time required totransfer the hydrate from the formation region (where it is at itsmaximum stability) to the dissociation region (where it is at itsminimum stability) ensures that a greater proportion of the hydrate willdissociate relatively high in the catch basin. This reduces the amountof mixing of fresh and residue water and increases the relativeproportion of fresh water that can be recovered.

Pumped hydrate slurry arrives in the dissociation/heat exchange basin ina concentrated form with little more than intergranular saline waterpresent. Care is taken to allow the saline water to separate downwardand fresh water upward so that there is a minimum of mixing. This isachieved by placing a slurry holder and fluid separator tank in theupper part of the dissociation/heat exchange chamber 438. This allowsthe negative buoyancy hydrate dissociation to take place so that salinewater is delivered to and collects in the lower part of the dissociationchamber 438, in which the slurry holder and fluid separator tank isplaced, without mixing with fresh water.

A preferred slurry holder and fluid separator consists of a fixed,wide-mouthed, upwardly open tank or tanks 450 (FIG. 14) that receive thehydrate slurry from above. Each tank holds the negatively buoyanthydrate from the hydrate slurry transfer system 446 and prevents it fromsinking toward the base of the dissociation chamber 438. The hydrateslurry is delivered by pipes 448 to a number of hydrate spreadersconsisting of vanes or rotating vanes designed to disperse the granularhydrate 468. The negatively buoyant hydrate separates while falling to ascreen shelf 470 in the tank. This allows saline water to sink throughthe screen shelf at the base of the circulating input water intercoolersystem 474, which transfers heat from the input water to thedissociating hydrate and feeds the cooled water downward to be treated.

A number of residual water delivery pipes 475 extend downward from thebase of this slurry holding tank, which allows heavier saline water toflow to the base of the vessel without disturbing the water surroundingthese pipes. Thus, even when the fresh water-saline water interface islocated in the vessel below the slurry holding tank, no mixing occursbetween the residue water purged from each input of hydrate slurrybecause of a physical separation. The main interface 477 (dashed line)between fresh and saline water will be located somewhere the lower partof the dissociation/heat exchange chamber 438, where saline waternaturally collects below fresh due to density differences. Saline wateris removed at the base of the chamber 480, and provision is also madefor gray water removal as at 482.

Multiple slurry holding tanks may be placed within a givendissociation/heat exchange chamber so that the flow of hydrate slurrycan be rapid enough to prevent clogging or freezing up of any one tank.Circulating input water may be passed first through one slurry holdingtank and then through another to minimize temperature of the input wateras it exits the dissociation/heat exchange chamber.

All fluids will find their relative positions according to naturalbuoyancy or through a process of fractionation. All internal piping inthe vessel can be fabricated from inexpensive plastic or other material.This method of fluid separation may also be installed in thedissociation/heat exchange section of the assisted buoyancy and pumpedsump embodiment shown in FIG. 8.

Preferably, the slurry pumping conduit 446 is constructed as a variablevolume pipe, in order to permit periodic pumping of hydrate withoutallowing the hydrate to settle or move upward slowly. Such a variablevolume pipe can be fabricated relatively easily by inserting a flexiblesleeve within the slurry pumping conduit 446 around which fluid canflood when the pressure within the liner is reduced.

The injection point 444 of the hydrate-forming liquid or gas, it will benoted, is positioned above the base of the column 440 so that in theevent of incomplete hydrate formation (which would result in theformation of assisted buoyancy hydrate), any excess gas which does notform hydrate (along with assisted buoyancy hydrate) will rise up theresidue fluid riser conduit 436. (Very little hydrate will escape withgas up the residue fluid riser conduit 436, and any such hydrate willhave dissociated prior to arriving at the top of the residue risersection. Therefore, the amount of fresh water “lost” by beingtransported by such hydrate will be minimal; recovery of that freshwater is not feasible; and accordingly no connection is provided betweenthe output of the residue fluid riser conduit 436 and thedissociation/heat exchange catch basin 438.)

For proper operation of this embodiment, flow rate controls such asconstrictors should be used to keep the rate of flow of fluid throughthe system low enough to keep solid hydrate from being swept up theresidue fluid riser conduit 436. Furthermore, the design of the hydrateformation and catch sump 434, as well as the lower portion of theresidue fluid riser conduit, should be designed to facilitate “clean”separation of the hydrate from the residue fluid. Accordingly, thehydrate formation and catch sump 434 is designed to impart lateralmovement to the residue fluid as well as to permit upward movementthereof. This causes the hydrate/residue fluid mixture to move initiallywith a relatively small upward component, which facilitates settling outof the hydrate and which is in contrast to the previously describedembodiments, which provide more vertically oriented fluid movement thatis comparatively turbid and which have poorer settling and separationcharacteristics.

In the embodiments described thus far, the weight of the column of watercreates the pressures required for hydrate formation. In theseembodiments, the minimum pressure depth at which hydrate is stable isfar greater than at sea level, where the pressure is one atmosphere.Accordingly, the hydrate begins to dissociate at relatively elevatedpressures.

Various ones of the embodiments described above may be modified so as tocollect the fresh water released from the hydrate and to capture thereleased gas at the region of the fractionation column where thedissociation takes place, rather than at the top of the column (surfacelevel; one atmosphere ambient pressure), with certain resultantadvantages. In particular, relatively large volumes of hydrate-forminggases and gas mixtures are required to desalinate large volumes ofwater. Therefore, if the gas is captured, processed for re-injection,and stored while maintained at elevated pressures (e.g., the pressure atwhich the hydrate begins to dissociate), the volume.of gas that must behandled will be much smaller than would be the case if the gas wereallowed to expand fully as it rises to the surface and pressure drops toatmospheric. Additionally, if the hydrate-forming gas is keptpressurized, raising its pressure to the pressure required for injectionin the hydrate-forming section requires far less recompression of thegas and hence is less costly.

A preferred embodiment 600 in which dissociation and gas capture andprocessing are controlled so as to be kept at elevated pressure isillustrated in FIG. 15. In this embodiment, a physical barrier 610extends across the fractionation column and blocks the upward movementof the hydrate slurry. The location of the barrier 610 depends on thestability limits of the particular hydrate-forming substance used, butwill be above the region of hydrate stability (i.e., at lesserpressure-depth). As the hydrate dissociates, the released gas forms apocket at trap 620 and enters a gas recovery and processing system 626while still at a pressure depth considerably greater than one atmospheresurface pressure. (The gas processing system 626 may contain means forliquefying certain gases.) The gas is processed and re-injected into thehydrate formation section 628 at 629 in the same manner as in thepreviously described embodiments, except the gas system is maintained atconsiderably higher pressure.

The hydrate dissociation section 630 extends downward to some particulardepth determined by the particular hydrate-forming gas being used.Because the hydrate dissociates under “controlled,” elevated pressure,the dissociation reaction will proceed generally more slowly than in theabove-described embodiments. Therefore, the heat exchanger 632 presentin the dissociation/heat exchanger section (as described in connectionwith previous embodiments) is designed to accommodate the particular,slower reaction rates. Input water 634 is passed through thedissociation/heat exchange section in heat exchanger 632 and is injectedinto the base of the desalination fractionation column at 636, as inpreviously described embodiments.

One or more fresh water bypass pipes 640 communicate with thedissociation region at a point 641 located above the fresh water/salinewater interface 644 but below the upper boundary 648 of the hydratestability field. The pipe(s) 640, which are screened or otherwiseconfigured to prevent hydrate from entering them, deliver fresh waterreleased from the hydrate to fresh water accumulation region 666. A graywater return pipe 650 allows denser, more saline gray water to flow backdown into the saline fluid below the fresh water/saline water interface644. More highly saline residual water and/or negatively buoyant hydrateis drawn from the sump 654 and processed or removed as at 658, as inpreviously described embodiments. Output fresh water, some of which maybe returned to the fluid removal section for purposes of washinginterstitial saline water as described above (not shown), is drawn offat 660, near the top of the fresh water accumulation region 666 and wellabove the physical barrier 610.

It is contemplated that the physical barrier 610, the fresh water andgray water return pipes 640, 650, and the heat exchanger in thedissociation/heat exchange section 630 may be configured such that theirpositions can be varied, thereby permitting different hydrate-formingliquids, gases, or gas mixtures to be used in the same installation. Thephysical barrier 610 and heat exchanger might be vertically adjustable,whereas a series of bypass and return pipes 640, 650 having differentinlet locations can be provided and opened and closed remotely usingsuitable inlet and outlet valves. In this manner, changing from onehydrate-forming substance to another can be effected very quickly andconveniently.

By holding and fully processing for re-injection the hydrate-forming gaswhile it is still under pressure, considerable economies of operationcan be achieved. The variation in the pressure of the liquid or gas,from that required for formation of the hydrate down to that at whichfresh water is released from the hydrate, can be kept to a minimum.This, in turn, minimizes the energy cost associated with pumping thecaptured hydrate-forming gas from above the dissociation/heat exchangesection back down to the hydrate-forming section at the base of theapparatus, particularly considering the fact that, percentagewise, thegreatest change of pressure in a hydraulic column (such as any of theabove-described embodiments) takes place in the upper portions of thecolumn. Moreover, the volume of the gas to be handled (and accordinglythe size of the gas handling equipment and facility) will also bereduced significantly.

As an alternative (not shown) to the configuration shown in FIG. 15, theupper part of the desalination fractionation column can be sealed andpressurized by means of an associated hydraulic standpipe, therebycausing pressures within the apparatus near the surface to be equivalentto the pressure-height of the standpipe. Where the standpipe isimplemented in tall structures (such as adjacent buildings near thedesalination facility), relatively high pressures can be created in thetopmost part of the dissociation/heat exchange section, which is atground level.

In further embodiments of the invention, the water may be desalinated orpurified in self-contained, mechanically pressurized vessels. Suchembodiments offer a number of distinct advantages, including the factthat the installations can be of various sizes and shapes to suit localconditions, containment constraints, and fresh water requirements.Moreover, whereas the previously described embodiments are relativelylarge-scale and therefore are of a fixed, permanent nature,self-contained, pressurized embodiments can be more temporary in naturein terms of their construction and their location. Individualpressurized installations can occupy relatively small spaces and producefresh water efficiently, even in low volumes. Such installations can befabricated at central manufacturing facilities and installed on sitewith a minimum of local site construction, which site might be abuilding or even a ship or other mobile platform.

A mechanically pressurized installation configured to use positivelybuoyant hydrate to extract fresh water from water is illustrated in FIG.16. Input water is pumped and pressurized from input pressure to theoperating system pressure by pump 704. The water enters the pressurizedhydrate formation and separation vessel 710 at water input 711, and asuitable, positively buoyant hydrate-forming substance is injected atinjection point 712. (Means 713 for liquefying certain gases areprovided where this is advantageous to the desalination process.)Positively buoyant hydrate 714 spontaneously forms and rises through theresidual water, as in previously described embodiments, to the top ofthe vessel 710 where it accumulates and concentrates.

The buoyant hydrate slurry is subsequently admitted into transfer andwashing section 720, and then into the dissociation/heat exchange vessel722. (Flow of the hydrate slurry is regulated by valves 734.) While inthe transfer and washing section 720, the hydrate may be washed of theresidual, intergranular saline fluid using fresh water 726 tapped fromthe fresh water output 728. More than one wash cycle may be used tocompletely flush residual fluid, although the number of washings willdepend on the effectiveness of separation through fractionation (whichmay vary for different gases and gas mixtures) and the nature of thecrystalline fraction of the slurry. In some cases, no washing may benecessary.

Pressure is maintained in the hydrate formation and separation vessel710 and in the dissociation/heat exchange vessel 722 by pressure balancereservoir systems 732 (one for each vessel), and movement of fluid fromone vessel to the other is controlled by varying pressure and using thein-line valves 734. The systems 732 each have a pressure-pump 733 and adiaphragm or gas-fluid interface 736, which are used to raise and lowerpressure in each vessel. Pressure in the vessels is controlled so thatthe hydrate remains stable as hydrate until it is finally collected andconcentrated at the top of the dissociation vessel 722. This is becausepremature dissociation will release considerable amounts of gas andtherefore will cause undesired mixing. Moreover, pressure conditions inthe dissociation vessel should be controlled to minimize turbulence inthe fluid-gas mixture and to promote efficient separation of saline andfresh water.

The dissociation and heat exchange vessel 722 may be constituted by anumber of linked, heat-exchanging devices in a number of different watertreatment chambers. The actual size, throughput, etc. will depend on theoverall system production rate which, in turn, will depend on thetemperature of the input water, the particular liquid, gas, or gasmixture used to form the hydrate, the rate at which heat can be removedfrom the system, etc. Fractionation, concentration, separation, drying,and re-use of the hydrate-forming gas takes place in the same manner asin the previously described embodiments. Additionally, heat produced byliquefying hydrate-forming gas can be absorbed and removed using heatexchangers containing residue or saline fluids.

It will be appreciated that the mechanically pressurized process isinherently less continuous than the previously described embodiments andis essentially a batch process. Pressure in the system is controlled soas to simulate the pressure variation in the previously describedembodiments: the water to be treated is pressurized and injected intothe apparatus, and then pressure is raised and lowered to control therate of the hydrate formation and dissociation reactions.

Mechanically pressurized embodiments provide increased versatility inthat pressures may be controlled to provide the optimum pressures forformation of hydrate and to control the rate of dissociation. Moreover,different liquids, gases, and gas mixtures can be used within the sameapparatus, and the same water can be processed more than once usingdifferent liquids, gases, and gas mixtures.

A further mechanically pressurized embodiment 800, which embodimentutilizes negatively buoyant hydrate to extract fresh water from water tobe treated, is shown in FIG. 17. Input water is pumped from inputpressure up to the operating system pressure and into the pressurizedhydrate formation and separation vessel 810 by pumps 804, and asuitable, negatively buoyant hydrate-forming gas is injected atinjection point 812. (Means 813 for liquefying certain gases may beprovided.) Negatively buoyant hydrate 814 spontaneously forms and sinksthrough the residual water, as described in connection with previouslydescribed negatively buoyant hydrate embodiments, and collects andconcentrates in gated sump isolation sections 816, which are opened andclosed to control passage of the hydrate therethrough.

As in the previously described mechanically pressurized embodiment,pressure is maintained in the system by pressure balance reservoirsystems 832 (one for each vessel), and movement of the fluid can becontrolled by varying the pressure in the system compartments. Pressurepumps 830 and diaphragms or gas-fluid interfaces 836 are used to raiseand lower pressure in each vessel independently.

As the hydrate slurry passes through the transfer and washing section820 and into the dissociation/heat exchange vessel 822, it may be washedof the residual, intergranular saline fluid with fresh water tapped fromthe fresh water output 828, which removes salt from the hydrate slurryprior to dissociation.

Subsequently, the hydrate is permitted to flow downward from thetransfer and washing section 820, and into the hydrate dissociation andheat exchange vessel 822, where is dissociates and fresh, gray, andsaline water are removed. Heat exchange between the input water and thedissociating hydrate slurry occurs as described in previous embodiments.Dissociation takes place under controlled pressure conditions tominimize turbulence in the fluid-gas mixture and to promote efficientseparation of saline and fresh water.

Preferably, a slurry holder and fluid separator tank 860 is provided inthe upper part of the dissociation/heat exchange vessel 822 and issimilar in construction to that described above and shown in FIGS. 13and 14. The tank 860 minimizes mixing of fresh and saline water byproviding a conduit for the residual saline water to sink to the bottomof the vessel, which conduit isolates the saline water from the lowerdensity fresh water.

As in the case of the mechanically pressurized, positively buoyanthydrate embodiment, the dissociation and heat exchange vessel 822 may beconstituted by a number of linked, heat-exchanging devices in a numberof different water treatment chambers. The actual size, throughput, etc.will depend on the production rate which, in turn, will depend on thetemperature of the input water, the particular liquid, gas, or gasmixture used to form the hydrate, the rate at which heat can be removedfrom the system, etc. Fractionation, concentration, separation, drying,and re-use of the hydrate-forming substance takes place in the samemanner as in the previously described embodiments.

Another embodiment 900, which embodiment provides greater versatility byusing either positively or negatively buoyant hydrate to extract freshwater from seawater or polluted water, is shown in FIG. 18. Pumps P andin-line valves 914 are provided throughout the system. Operation,depending on the particular hydrate-forming substance used, is asdescribed in the pressurized vessel installations using eitherpositively or negatively buoyant gas hydrate.

This embodiment is particularly useful where the gas or gas mixturesupply is uncertain as a variety of gases may be used. Embodiments ofthis type could be useful in disaster relief or in expeditionarymilitary activity, or at any place where a temporary supply of freshwater is required without a significant construction requirement. Thisembodiment contains all the attributes of both the positive and negativebuoyancy hydrate, mechanically pressurized desalination fractionationembodiments, including use of fresh water 926 from the fresh wateroutput 928 to flush residual saline water. Multiple liquid or gasinjection points 908 are provided, as well as provision for handlingeither positively or negatively buoyant hydrate. In particular, multiplepumping units P and fluid control valves 914 are provided to direct theflow of fluids and hydrate slurries in fluid control and washing units916 and hydrate slurry control units 918. The gas processing system 944includes means for liquefying certain recovered gases and gas mixtures.

As in the above-described embodiments in which the weight of the columnof water generates the requisite pressures, any of the mechanicallypressurized vessel installations may be simplified by feeding the inputwater into the system without passing it through the dissociationsection for heat exchange. More artificial refrigeration will need to beprovided, but operation will otherwise be the same as for the positiveand negative buoyancy hydrate embodiments shown in FIGS. 16 and 17 andthe “combined” pressurized apparatus as shown in FIG. 18.

As noted above, mechanically pressurized embodiments of the inventionmay be extremely mobile. In the case of a ship-borne installation (FIG.19), for example, water to be treated is processed as described inprevious embodiments, but in a smaller and more compact installationbuilt right on board the ship 1000.

Negatively buoyant hydrate formed from a liquid or gas (such as carbondioxide) that is non-combustible at the pressures and temperatures ofthis system and the surrounding ambient conditions is preferred,especially where installations are placed in ships or where the handlingof combustible gases constitutes a hazard.

In the ship-borne embodiment, some of the heat produced by the hydrateformation reaction is extracted by heat exchangers in the hydrateformation and concentration vessel, which is possible because of theimmediate access to seawater. Further heat is extracted from the hydrateslurry in the hydrate slurry transfer system. This pre-dissociation heatextraction maximizes the cooling effect of the hydrate dissociationbecause removing heat in addition to that removed with the residualtreated fluid allows dissociation to begin with the hydrate slurry at alower temperature than would exist otherwise. Thus, the fresh waterproduced will be significantly cooled. This cooled water can be used toabsorb heat and hence can be used to provide refrigeration orair-conditioning. The fresh water is treated as described in previousembodiments, and warmed residual water may be used as a low-grade heatsource (although it is more likely to be pumped back to the sea).

Installation aboard a ship is ideal for the mechanically pressurizedhydrate fractionation method of desalination. This is because theresidual treated water can be returned to the sea immediately, therebymaximizing efficiency of the heat-removal process. The water intake forthe desalination ideally would be placed as low on the keel 1010 of theship as possible to separate intake and residual water return and tominimize uptake of pollutants, which in the case of oil-based productsand many industrial chemicals either float or are usually found inincreasing proportion closer to the sea surface.

Aboard ship, the return fluid can have multiple outlets 1020, whichallows it to be returned to the sea closer to the surface where thewarmer water will float well.away from the water intake. In addition,movement of the ship creates considerable turbulence which will promotemixing of the residual water and near-surface water when the ship isunder way. When the ship is tied up, water from a shore source can beused or the system can be recycled with fresh water to minimize residualwater return, and the desalination fractionation system can be operatedat a minimal level, i.e., at a level just sufficient for the thermalbalance required for normal operation to be attained quickly. Where theship is moored or otherwise maintaining a static position, the residualwater can be returned to the sea directly. Wind and tide can be takeninto consideration to select the return outlet utilized so as tominimize environmental impact and allow the residual water to be carriedaway from the ship most efficiently.

Similar compact installations can be fabricated as pre-packagedcomponents that can be airlifted or easily flown and trucked to aparticular site—for instance, immediately following a disaster such asan earthquake—and assembled rapidly. Where temporary or mobileinstallations are operated, more compact versions of the intake,outfall, and gas processing apparatus similar to that described for FIG.1 are employed. These can be specially designed for light weight, easeof deployability, and ability to operate in a variety of conditions.Power generating units or power cables suitable for drawing electricityfrom any inshore powerboat or other supply are also part of the mobileapparatus, and possibly also part of larger temporary facilities.

Pressurized vessel desalination fractionation installations can also bemounted on pallets for shipment in aircraft or ships or in standardcommercial shipping containers (for which cargo handling equipmentexists world-wide) to facilitate air and road travel. They can bemounted on vehicles or set up on a pier, or anywhere near seawater orother water to be desalinated or purified.

Finally, once the seawater has been cycled through the pressurizedvessel desalination fractionation column and downstream processingapplications a desired number of times, the residual seawater is simplypumped back to sea or retained for those who desire it.

As noted above, one reason carbon dioxide is an ideal gas to use forhydrate desalination or purification is that it is extremely common and,in fact, is typically found in industrial waste products—particularly inthe exhaust gases produced when burning fossil fuels. (For the mostpart, the exhaust produced when burning fossil fuels typically containswater vapor, hydrogen sulfide, carbon dioxide, carbon monoxide, andnitrous oxides (NO_(x)) and nitrogen which passed through the combustionprocess in addition to soot and other particulate waste matter.

When using industrial exhaust as the source of carbon dioxide fordesalinating or purifying water—such purification may be the primaryobjective of the process, e.g., to produce potable water, or it may besimply a means for capturing carbon dioxide from the waste gas emissionsfor purposes of reducing “green house gases” and their associatedenvironmental harm, with the production of fresh water being a “side”benefit—certain provisions for pre-processing the gas must be made. Inparticular, an installation 1100 for simultaneously capturing carbondioxide from industrial waste gases and producing desalinated orpurified fresh water is illustrated in FIG. 20. Exhaust gas 1102produced by the combustion of fossil fuels in industrial plant 1104 isfed through dry processing/preprocessing means 1105. The raw exhaust gasis pretreated using filters, absorbents, electrostatic means, chemicalsorption techniques, and/or catalysis to remove most of the non-carbondioxide components. The exhaust gas must also be cooled substantially inorder for the carbon dioxide hydrate to form, and the preprocessingmeans 1105 may include means 1106 for such cooling, e.g., heatexchangers through which the exhaust gas flows.

Before being introduced into hydrate formation vessel 1111, the exhaustgas passes through washing/hydrate-forming prechamber 1108. Prechamber1108 may be a largely water-filled chamber that can be pressurized. Whenthe prechamber 1108 is, in fact, so pressurized, the exhaust gas passesthrough the water in the chamber under pressure conditions sufficient toform hydrogen sulfide hydrate. As illustrated in FIG. 21, hydrogensulfide hydrate will form at pressure/temperature conditions well abovethose required for carbon dioxide hydrate to form, and therefore themajority of the hydrogen sulfide components of the exhaust gas notremoved in the previous, preliminary processing can be removed byforming hydrogen sulfide hydrates in the prechamber 1108 and thenremoving it, e.g., in a solid hydrate flush.

The washing residue from the exhaust gas will consist of solids formedfrom soot, partially combusted hydrocarbons, small amounts of metals andsalts, and, when the prechamber 1108 is suitably pressurized and thereis hydrogen sulfide in the exhaust gas, hydrogen sulfide hydrate. (Thesolids will constitute only very small proportions of the exhaust gas.)When it is produced, the hydrogen sulfide hydrate subsequently willdissociate into hydrogen sulfide and water, with the attendantproduction of sulfuric acid and other hydrogen-oxygen-sulfur compounds,and along with the remaining solid waste will constitute hazardousmaterial in concentrated form which needs to be disposed of usingappropriate means that will be known to those having skill in the art.

The density of hydrogen sulfide hydrate, which has not been studiedextensively, has been calculated to be 1.05 g/cc and has been measuredat between 1.004 g/cc at 1 Mpa and 1.087 g/cc at 100 Mpa. Thus, itsdensity is very close to that of fresh water. Therefore, if theprechamber 1108 is filled with water, the hydrogen sulfide hydrate willnot separate from the water in the prechamber 1108 by sinking,particularly when large volumes of gas are rising quickly through theprechamber. Under conditions of low turbidity with gas bubbles risingthrough the water bath, the hydrogen sulfide hydrate will tend to risenear the surface of the water in the prechamber 1108, where it willaccumulate and form a hydrate-rich layer through which the exhaust gasbubbles. On the other hand, because the large volume of exhaust gasrising through the water in the prechamber 1108 will cause extremeturbidity, the hydrogen sulfide hydrate is unlikely to separate outnaturally by means of gravity fractionation. Therefore, in the case of awater-filled prechamber 1108, the prechamber will have to be flushedperiodically, which precludes operating the process on a continuousbasis.

Alternatively, if the prechamber 1108 is pressurized and largelygas-filled (which actually is preferred), a spray of water 1109 takenfrom the heat exchange system 1107 can be used to wash and cool theexhaust gas efficiently while allowing hydrogen sulfide hydrates toform. The water spray fills all but the lower part of the prechamberwith a mist of droplets that fall to the bottom of the vessel. (Waterevaporating out of the mist and subsequently passing into the hydrateformation vessel 1111 would simply become part of the fresh waterproduct.) Solid matter and any hydrogen sulfide hydrate that forms isseparated from the wash water, which is heated as it cools the exhaustgas, by means of separation filter 1110. (A separation filter 1110 mayalso be used in an embodiment in which the prechamber 1108 ispressurized and water-filled, as described immediately above, to filterthe wash water.) The solid waste will consist of concentrated hazardousmaterials that must be disposed of according to recognized practices,and the heated wash water is passed back into the heat exchange system1107.

After it has been cooled sufficiently by intercoolers 1113 for hydrateto form spontaneously under operational pressure and temperatureconditions, e.g. by processes as described previously for cooling theinput water, the fully preprocessed exhaust gas 1102′ then passes intohydrate formation vessel 1111. The hydrate formation vessel 1111 ispressurized and is configured essentially the same as the pressurizedhydrate formation vessel shown in FIG. 17 or, to the extent it isconfigured for the production of negatively buoyant hydrate, thepressurized vessel shown in FIG. 18. As is the case with those twoembodiments, saline or polluted water and the exhaust gas containing thecarbon dioxide are introduced into the hydrate formation vessel 1111 andpressure and temperature conditions are controlled so as to form carbondioxide hydrate, as described previously. (As in the previousembodiments, this will be done on a batch basis as opposed tocontinuously.) Residual gases such as nitrogen oxide are purged from thesystem.

The carbon dioxide hydrate 1112 is then transferred to pressurizedhydrate dissociation vessel 1114, which may be located below the hydrateformation vessel 1111 as shown in FIG. 17 or, as shown in FIG. 20,adjacent to the hydrate formation vessel (similarly to as shown in FIG.18). The hydrate dissociation vessel 1114 is shown in greater detail inFIG. 22. It is generally similar to the pressurized hydrate dissociationvessels shown in FIGS. 17 and 18, but differs in that a heating element1115, which preferably constitutes part of the system's heat exchangesubsystem 1107, is provided along with an additional outlet 1120 for theremoval of liquid carbon dioxide from the vessel. Concentrated carbondioxide hydrate slurry is introduced into the vessel 1114, as describedpreviously, and the introduction of it into the vessel is controlled bymeans of valve mechanism 1122 so that the carbon dioxide hydrate slurryis introduced on a batch basis. (This is necessary because the pressurein the hydrate dissociation vessel will rise to greater than that in thehydrate formation vessel 1111.) Alternatively, in-line slurry pumps canbe used to maintain higher pressure in the downstream hydratedissociation vessel. The hydrate is held in the upper part of the vessel1114 by a screen 1113 within the tray 1124, where the hydratedissociates.

By controlling the temperature and pressure conditions within the vessel1114, dissociation of the carbon dioxide hydrate is managed so as toproduce fresh water (as described above in connection with the previousembodiments) and liquid carbon dioxide (in contrast to gaseous carbondioxide). In particular, as shown in FIG. 23, when the hydrate isintroduced into the dissociation vessel 1114, it is introduced underpressure and temperature conditions in the hydrate stability field inthe vicinity of point A, i.e., under conditions at which the hydrateremains stable. (Pressure within the dissociation vessel 1114 may becontrolled by means of a pneumatic standpipe 1126 by admitting an inertgas into the standpipe using valve 1128.) The temperature within thedissociation vessel 1114 may then be permitted to rise, e.g., byabsorbing heat from the surroundings or, more preferably, is caused torise by actively adding heat removed from the hot exhaust gas back intothe system via heat exchanger 1115. Alternatively, the temperature maybe caused to rise advantageously by passing the hot exhaust gas aroundthe dissociation vessel 1114 through conduits (not shown) before theexhaust gas is p reprocessed in means 1106 and 1108.

As heat being input from the heat exchange system causes the temperatureto rise, the system moves to the right along P-T path 1130 and crossesphase boundary 1132, at which point the hydrate dissociates into carbondioxide gas and fresh water. Because the vessel is sealed, however, asthe hydrate continues to dissociate, the pressure and accordingly thetemperature continue to rise. As the temperature and pressure increase,the system continues to move toward the right along P-T path 1130, butthe pressure rises at a sufficient rate that the P-T curve 1130 alongwhich the system moves crosses below the carbon dioxide liquidus, andthe carbon dioxide gas condenses to carbon dioxide liquid, asillustrated at point B. (Point B represents just an example of thesystem temperature/pressure conditions at which dissociation iscomplete; the exact conditions are less important than making sure thatthe final pressure/temperature conditions within the hydratedissociation vessel lie within the field of stability for liquid carbondioxide.) In operation, the system may pass relatively quickly throughthe portion of the phase diagram in which carbon dioxide gas exists. Inthose instances, the hydrate will essentially dissociate directly intocarbon dioxide liquid and pure water.

Alternatively, by controlling the pressurization of the dissociationvessel 1114 using the gas valve 1128, pressure can be increasedsufficiently fast so that the carbon dioxide never enters the gas phase.The system will then move along P-T curve 1130′ through the lower,hydrate stability zone 1132 and directly into the liquid carbondioxide/water zone 1134. In either case, however, the result is theessentially immediate production of liquid carbon dioxide and freshwater.

Non-hydrate-forming components of the exhaust gas, such as nitrogenoxides and nitrogen, will be released from the system throughvalve-controlled purge system 1150 once the liquid carbon dioxide andwater are removed from the system.

(As yet another, less preferred alternative, if pressure in the systemis controlled so as not to rise as high as in these two embodiments,gaseous carbon dioxide instead of liquid carbon dioxide will bereleased. In that case, the gaseous carbon dioxide can be removed fromthe hydrate dissociation vessel 1114 at the relatively high gaspressures of the vessel but below those required for hydrate stability(e.g., two hundred to three hundred atmospheres pressure) and compressedto liquid carbon dioxide at relatively low temperature using industrystandard apparatus and methods. Compressing the pressurized carbondioxide from the already pressurized gaseous state to the liquid statewould still be easier—and hence less expensive—than the case wheregaseous carbon dioxide is compressed to liquid carbon dioxide fromapproximately normal atmospheric pressure.)

As illustrated in FIG. 24, at the pressure and temperature conditionsthat exist at the dissociation end point B, the liquid carbon dioxidewill be positively buoyant with respect to the fresh water produced fromdissociation of the carbon dioxide hydrate. Moreover, the liquid carbondioxide and the fresh water are essentially immiscible. Accordingly, theliquid carbon dioxide will float on top of the fresh water released bythe hydrate, with an interface 1152 (FIG. 22) between the two. The freshwater is then extracted by valve-controlled outlet 1154, and the liquidcarbon dioxide is extracted by valve controlled outlet 1120. The freshwater will be saturated with dissolved carbon dioxide and may containmicro-droplets of liquid carbon dioxide that can be withdrawn underpressure. The fresh water may then be used for any desired purpose,e.g., drinking water, and the carbon dioxide may be transported awayquite conveniently in its liquid form and either used in a variety ofexisting commercial applications or disposed of (e.g., by being pumpedto great ocean depths for sequestration.) Any residual saline water,which will settle to the very bottom of the system, is extracted via avalve-controlled outlet 1156, as described previously, and disposed of.Preferably, even though the carbon dioxide and water are essentiallyimmiscible, a conduit 1158, arranged to carry the more dense fresh waterdown below the less dense liquid carbon dioxide, is provided extendingfrom the bottom of the tray 1124 so that the separated water can flowdownward without having to flow through the carbon dioxide.

Finally, FIG. 22 shows the system input water as passing through thedissociation vessel 1114 as in the previously described embodiments. Itwill be appreciated, however, that because heat is being added to thesystem through the heat exchange system 1007 so as to move thetemperature to the right in the phase diagram shown in FIG. 23, lessheat will be absorbed out of the input water by the endothermicdissociation of the hydrate, and therefore the input water will not bechilled to the same extent as in the previously described embodiments.Accordingly, additional supplemental cooling may be necessary in orderfor the hydrate to be formed.

Finally, although just a single one of each is shown, more than one dryprocessing/preprocessing apparatus 1105, prechamber 1108, hydrateformation vessel 1111, and/or hydrate dissociation vessel 1114 may beemployed in a single installation. For example, it may be desirable oreven necessary to cycle the exhaust gas a number of times prior tointroducing it into the hydrate formation vessel 1111 in order to reducethe levels of non-carbon dioxide materials which otherwise might causethe water to taste poor.

Although particular and specific embodiments of the invention have beendisclosed in some detail, numerous modifications will occur to thosehaving skill in the art, which modifications hold true to the spirit ofthis invention. Such modifications are deemed to be within the scope ofthe following claims.

I claim the following:
 1. A method of 1) capturing carbon dioxide fromexhaust gas containing carbon dioxide and produced by an exhaustgas-producing plant and 2) producing purified water generallyconcurrently therewith, said method comprising: providing said exhaustgas directly from said exhaust gas-producing plant to a waterpurification installation; at said water purification installation,mixing said exhaust gas, or an exhaust gas remainder produced at saidwater purification installation by washing said exhaust gas to removenon-carbon dioxide components therefrom, with saline or otherwisepolluted water under pressure and temperature conditions conducive tothe formation of carbon dioxide hydrate such that carbon dioxide hydrateis formed from carbon dioxide contained within said exhaust gas andwater molecules contained within said saline or otherwise pollutedwater; causing or allowing said carbon dioxide hydrate to be subjectedto conditions under which it dissociates and allowing said carbondioxide hydrate to dissociate into said carbon dioxide and said watermolecules; collecting carbon dioxide released upon dissociation of saidcarbon dioxide hydrate; and collecting fresh water released upondissociation of said carbon dioxide hydrate; whereby carbon dioxide fromsaid exhaust gas-producing plant is captured or sequestered to preventits release into the atmosphere and purified water is produced generallyconcurrently with said capture or sequestration.
 2. The method of claim1, further comprising pretreating the exhaust gas at said waterpurification installation using one or more means selected from thegroup consisting of filters, absorbents, electrostatic means, chemicalsorption techniques, and catalysis to remove non-carbon dioxidecomponents from said exhaust gas.
 3. The method of claim 1, furthercomprising cooling said exhaust gas at said water purificationinstallation.
 4. The method of claim 3, further comprising transferringheat removed from said exhaust gas by said cooling to said carbondioxide hydrate to facilitate dissociation thereof.
 5. The method ofclaim 1, further comprising transferring heat from said exhaust gas tosaid carbon dioxide hydrate to facilitate dissociation thereof.
 6. Themethod of claim 1, wherein, during said washing to produce said exhaustgas remainder, hydrate of a component of said exhaust gas other thansaid carbon dioxide is formed.
 7. The method of claim 1, wherein saidcarbon dioxide hydrate is allowed to dissociate while pressure andtemperature conditions are controlled such that said carbon dioxidehydrate dissociates along a P-T pathway that causes said carbon dioxidehydrate to release said carbon dioxide directly or essentially directlyas liquid carbon dioxide.
 8. The method of claim 1, further comprisingdelivering said fresh water to a person or animal for drinking.
 9. Amethod of producing liquid carbon dioxide, said method comprising:forming carbon dioxide hydrate from gaseous carbon dioxide and waterunder pressure and temperature conditions conducive to the formation ofsaid carbon dioxide hydrate; causing said carbon dioxide hydrate to besubjected to conditions under which it dissociates and causing saidcarbon dioxide hydrate to dissociate directly or essentially directlyinto liquid carbon di oxide and water by controlling pressure andtemperature conditions such that when said carbon dioxide hydratedissociates, said carbon dioxide hydrate dissociates along a P-T pathwaythat causes said carbon dioxide hydrate to release said carbon dioxidedirectly or essentially directly as liquid carbon dioxide.
 10. Themethod of claim 9, wherein said gaseous carbon dioxide comprises acomponent of exhaust gas and said carbon dioxide hydrate is formed bymixing said exhaust gas, or an exhaust gas remainder produced by washingsaid exhaust gas to remove non-carbon dioxide components therefrom, withsaid water.
 11. The method of claim 10, further comprising pretreatingthe exhaust gas using one or more means selected from the groupconsisting of filters, absorbents, electrostatic means, chemicalsorption techniques, and catalysis to remove non-carbon dioxidecomponents from said exhaust gas.
 12. The method of claim 10, furthercomprising cooling said exhaust gas.
 13. The method of claim 12, furthercomprising transferring heat removed from said exhaust gas by saidcooling to said carbon dioxide hydrate to facilitate dissociationthereof.
 14. The method of claim 10, further comprising transferringheat from said exhaust gas to said carbon dioxide hydrate to facilitatedissociation thereof.
 15. The method of claim 9, further comprisingdelivering water released by said dissociation to a person or animal fordrinking.
 16. The method of claim 10, wherein, during said washing toproduce said exhaust gas remainder, hydrate of a component of saidexhaust gas other than said carbon dioxide is formed.